Robert Zeithammer (2014) show that outcome-based social preferences and strategic considerations to keep the seller in the market can explain why and how much buyers voluntarily pay to a PWYW seller. They find that PWYW can be viable in a monopolistic market, but is less successful as a competitive strategy because it does not drive traditional posted-price sellers out of the market. Instead, the existence of a posted-price competitor reduces buyers' payments and prevents the PWYW seller from fully penetrating the market. When given the choice, most sellers opt for setting a posted price rather than a PWYW pricing strategy. Another PWYW experiment looked at determinants for the price chosen by consumers of the application iProduct, which provided tutorials and lessons for potential application developers on the App Store (iOS). The application was offered as free with in-app purchases, including a gratuity mechanism that allowed users to pay/donate what they wanted for the projects included in the app. The study tested the significance of four determinants in deciding the PWYW price paid by consumers: fairness (proper compensation to the seller), loyalty to the seller, price consciousness (focus on paying a low price), and usage (how much the consumer will use the product). The study found that price consciousness negatively influenced the price paid, while usage and loyalty positively influenced the price paid for the product. Fairness was found to have no significant effect. Further research focused on the long-term perspective of pay what you want. A study conducted by researchers of the Ruhr-University of Bochum examines repeated transactions in a pay what you want environment. By using latent growth modeling they find that the average price paid decreases significantly; yet the decrease in price paid reduces with every transaction. They further show customers' preference for fairness and price conscientiousness influence the steepness of the individual price curves. A broad review of the literature on PWYW and related forms of voluntary payment (tipping, donations, and gifts) by Natter and Kaufmann, published in 2015, examines many relevant factors as they relate to voluntary pricing strategies. These factors include product characteristics, consumer-related characteristics, situational variables, relational techniques, and reference prices. The review also addresses economic and communicative success, and underlying market motives. Enhanced forms There are several changes to the PWYW model which can improve its profitability while maintaining its buyer appeal. Ex post pricing One simple enhancement is to shift the time